Bi-stable switches are switches that are capable of remaining in one of two positions or stable states without requiring energy input to maintain those states. The stable states often correspond to the function of the switch. For example, switches may be used to open an electrical circuit in one stable position and to close the circuit in a second stable position. Correctly and consistently positioning and holding a switch at its stable states and switching between the stable states is often a critically important component of the switch design. This ensures that the switch functions reliably for its intended purpose. For example, in the case of an electrical switch, where the position of the switch governs whether an electrical circuit is open or closed, correctly orienting and holding the switch at the selected stable position is important to ensure that an “ON” switch remains in the on position and an “OFF” switch remains in the off position until moved by the user. For the same reason, it is also important that the switch does not move past or over the desired stable state position. Rather, it is important for movement of the switch to cease precisely at the stable positions.
Typically, transition between the stable states of a bi-stable switch is achieved by a single actuation of energy. The type and magnitude of actuation energy for transition between stable states can be modified according to the type and function of the switch as well as the environment in which it is intended to be operated. For example, a bi-stable switch in a mechanical system may have different requirements for type and magnitude of transition or actuation energy from switches in electrostatic, magnetic, thermal, and pneumatic applications.
Historically, there have been several methods for sustaining the stable state, including using latches, living hinges, hooks, and residual stress. Alternatively, in buckling-type, bi-stable mechanisms, the device is located and held at the stable states without any further input of energy as a result of residual stress in a structural member of the device.
Buckling is an instability or failure mode that may occur when a structural member is placed under sufficient compressive stress. The compressive stress required for buckling to occur is known as the “critical load.” At the critical load, the structure becomes unstable and the introduction of the slightest force will cause the structure to buckle. Buckling is characterized by a sudden transverse deflection (“jumping”) of the structural member once the critical load has been reached from one configuration to another configuration. The critical load is inversely related to the unsupported length of the structural member. As the unsupported length increases, the load necessary for buckling to occur decreases. Likewise, as the unsupported length decreases, the load necessary for buckling to occur increases. Accordingly, the critical load may be adjusted by increasing or decreasing the unsupported length of the structural member.
The term “structural member” refers to a portion of the switch that moves between a first stable position and a second stable position. The structural member of a bi-stable, buckling-type switch can be in the form of a beam that bends in one direction to the first stable position and bends in a second direction to the second stable position. As discussed above, this type of device utilizes a mode of failure (i.e., buckling) that results from structural instability from forces, such as compression acting on a structural member, to create the necessary residual stress. For example, in some cases, residual stress is the result of material deformation. The residual stress is confined so that the resulting pressure holds the structural member at a stable state with no further input of energy required.
Typical buckling-type, bi-stable mechanisms require the structural member to be fixed at two points to enable the creation of residual stress in the structural member and to correctly and reliably move between the two stable states. More specifically, many bi-stable devices include a structural member (i.e., a beam) where each end is fixed and a transition force is applied transversely at some location between those fixed points or is applied axially at one end of the device and along an axis that runs between both fixed points. In either case, once a sufficient transition force is applied, the structural member buckles and then moves from one stable position to another stable position. As discussed above, this change in stable states is typically manifested by the beam moving from bowing in one direction to bowing in the opposite direction.
However, having a structural member fixedly mounted at both ends is not always desired. In certain instances, for example, it may desirable to have a structural member having a free end to simplify the manufacturing or operation of the device. Another problem often caused by fixing both ends of a beam in buckling-type, bi-stable device is instability. In a flexible beam having both ends fixedly mounted, the stable position is not symmetric. Moving a portion of the beam past the mid-point between the two bowed positions does not guarantee that the beam will move to and remain at the second stable bowed position. In fact, it may very easily return back to the original stable position. Thus, switching a beam between stable positions, where both ends of the beam are fixed, can sometimes be inconsistent.
Additionally, overloading and failure of the beam often occurs when both ends are fixed. Typically, when manufacturing a bi-stable switch, the structural member is fabricated so that it is longer than the distance between the two fixation points. Thus, the beam is placed under compression in order to provide the bowed shape and to fix both ends at the fixation points. This preloading combined with the additional transitional forces may cause the beam to be overloaded and to fail prematurely. Thus, the useful life of the beam fixed at both ends may be limited due to overloading.
For the reasons discussed above, reliably or consistently moving to the same stable position is an important characteristic of many bi-stable devices. Certain prior devices have required surface etching or other types of surface manipulation to repeatedly achieve stable state positioning with accuracy. Other devices require additional components, such as hinges, etc., to enable reliable movement between stable positions However, these additional features increase the cost and complexity of the device and may also cause the functional life of the product to be shortened.
Accordingly, what is needed is a bi-stable switch that is easier to manufacture and to operate and that may be consistently and repeatedly positioned in two stable configurations.